Question 1(a) [3 marks]

What is modulation? What is the need of it?

Answer: Modulation is the process of varying one or more properties (amplitude, frequency, or phase) of a high-frequency carrier signal according to the instantaneous value of a lower frequency message signal.

Need for modulation:

Antenna size reduction: Allows practical antenna size (λ/4)
Multiplexing: Enables multiple signals to share same medium
Interference reduction: Shifts signal to suitable frequency band
Range extension: Increases transmission distance

Mnemonic: "AMIR" - Antenna, Multiplexing, Interference, Range

Question 1(b) [4 marks]

Derive the expression for DSBFC of AM wave.

Answer: DSBFC (Double Sideband Full Carrier) AM wave derivation:

Mathematical derivation:

Carrier signal: c(t) = Ac cos(ωct)
Message signal: m(t) = Am cos(ωmt)
AM signal: s(t) = Ac[1 + μm(t)]cos(ωct)
Where μ = modulation index = Am/Ac

Substituting message signal: s(t) = Ac[1 + μ cos(ωmt)]cos(ωct) s(t) = Ac cos(ωct) + μAc cos(ωmt)cos(ωct)

Using trigonometric identity: cos(A)cos(B) = 1/2[cos(A+B) + cos(A-B)]

Final expression: s(t) = Ac cos(ωct) + (μAc/2)[cos((ωc+ωm)t) + cos((ωc-ωm)t)]

Diagram:

    ^
    |    Carrier
Ac  |    /|\
    |   / | \
    |  /  |  \
    | /   |   \
    |/    |    \
    +-----+-----+----> f
         fc

    ^
    |                LSB   Carrier   USB
    |                 |      |       |
Pam |                 |      |       |
    |                 |      |       |
    |                 |      |       |
    |                /|\    /|\     /|\
    +---------------+-+-----+------+-+----> f
                 fc-fm     fc    fc+fm

Question 1(c) [7 marks]

Classify Noise signal and explain flicker noise, shot noise and thermal noise.

Answer:

Noise Classification:

Type	Source	Characteristics
External Noise	Environmental sources	Outside communication system
Internal Noise	Components	Generated within system

Types of internal noise:

Flicker Noise:
- Source: Occurs in active devices
- Characteristics: Inversely proportional to frequency (1/f)
- Effect: Dominant at low frequencies
Shot Noise:
- Source: Random electron flow across junctions
- Characteristics: Independent of frequency (white noise)
- Effect: Random current fluctuations in diodes/transistors
Thermal Noise:
- Source: Random motion of electrons due to temperature
- Characteristics: Present in all conductors, resistors
- Formula: Pn = kTB (k=Boltzmann constant, T=temperature, B=bandwidth)
- Effect: Sets noise floor in receivers

Mnemonic: "FST" - Flicker decreases with Frequency, Shot is from electron flow, Thermal depends on Temperature

Question 1(c) OR [7 marks]

Describe EM wave also write at least one application of different band of spectrum.

Answer:

EM Wave: Electromagnetic waves are energy propagating through space as time-varying electric and magnetic fields, traveling at speed of light (3×10⁸ m/s).

Characteristics:

Transverse waves with E and H fields perpendicular to each other
No medium required for propagation
Described by wavelength (λ) and frequency (f)
Relation: c = f × λ

EM Spectrum and Applications:

Frequency Band	Frequency Range	Application
ELF	3Hz-30Hz	Submarine communication
VLF	3kHz-30kHz	Navigation systems
LF	30kHz-300kHz	AM broadcasting
MF	300kHz-3MHz	AM radio broadcasting
HF	3MHz-30MHz	Shortwave radio
VHF	30MHz-300MHz	FM radio, TV broadcasting
UHF	300MHz-3GHz	TV, mobile phones, WiFi
SHF	3GHz-30GHz	Satellite communication, radar
EHF	30GHz-300GHz	Millimeter wave communication
Infrared	300GHz-400THz	Remote controls, thermal imaging
Visible	400THz-800THz	Fiber optic communication
Ultraviolet	800THz-30PHz	Sterilization, authentication
X-Rays	30PHz-30EHz	Medical imaging
Gamma Rays	>30EHz	Cancer treatment

Diagram:

      +----------------+----------------+----------------+----------------+
      |                |                |                |                |
Radio   Microwave    Infrared       Visible         Ultraviolet     X-ray    Gamma
      |                |                |                |                |
      +----------------+----------------+----------------+----------------+
  Increasing Frequency →
  Decreasing Wavelength →

Mnemonic: "RMIUXG" - Radio, Microwave, Infrared, Ultraviolet, X-ray, Gamma

Question 2(a) [3 marks]

State advantages of SSB over DSB.

Answer:

Advantages of SSB over DSB:

Parameter	SSB Advantage
Bandwidth	50% less bandwidth requirement
Power	Power saving of 83.33%
Transmitter	Less power amplification needed
Receiver	Simpler design without phase distortion
SNR	Better signal-to-noise ratio
Fading	Less susceptible to selective fading

Mnemonic: "BP TRFS" - Bandwidth, Power, Transmitter, Receiver, Fading, SNR

Question 2(b) [4 marks]

Explain generation of FM wave using FET reactance modulator.

Answer:

FET Reactance Modulator:

Working principle:

Uses FET as voltage-controlled reactance
Changes effective capacitance based on modulating signal
Connected across LC tank circuit of oscillator

Circuit operation:

Modulating signal applied to gate of FET
FET drain-source resistance varies with gate voltage
Capacitive reactance changes with modulating signal
Oscillator frequency deviates with input signal

Diagram:

      +-----|>|-----+
      |             |
      |             C
      |             |
    V_in          +---+
      |           |FET|
      +-----R-----|   |
                  +---+
                    |
                   LC
                 Circuit

Key features:

Simple design: Fewer components than other modulators
Linearity: Good for wide-band FM generation
Stability: Temperature stable compared to varactor diodes

Mnemonic: "LOVE FM" - LC Oscillator with Voltage-controlled Element for FM

Question 2(c) [7 marks]

Derive the equation for total power in AM, calculate percentage of power savings in DSB and SSB.

Answer:

Power in AM signal:

For AM signal s(t) = Ac[1 + μcos(ωmt)]cos(ωct)

Total power calculation:

Power in carrier: Pc = Ac²/2
Power in sidebands: Ps = μ²Ac²/4 (total for both sidebands)
Total power: Pt = Pc + Ps = Ac²/2 × (1 + μ²/2)

For 100% modulation (μ=1):

Pt = Pc × (1 + 1/2) = 1.5 × Pc
Carrier power = 66.67% of total
Sideband power = 33.33% of total

Power savings:

In DSB-SC:
- Carrier is suppressed
- Power saved = 66.67%
In SSB:
- Carrier + one sideband suppressed
- Power saved = 66.67% + 16.67% = 83.33%

Comparative Table:

Modulation	Carrier Power	Sideband Power	Total Power	Power Saving
AM (μ=1)	100%	50%	150%	0%
DSB-SC	0%	50%	50%	66.67%
SSB	0%	25%	25%	83.33%

Mnemonic: "CST" - Carrier power, Sideband power, Total power

Question 2(a) OR [3 marks]

Draw and explain Time domain and Frequency domain display of AM wave.

Answer:

Time Domain and Frequency Domain Display of AM Wave:

Time Domain:

Shows amplitude variations over time
Envelope follows modulating signal
Maximum amplitude: A₁ = Ac(1+μ)
Minimum amplitude: A₂ = Ac(1-μ)
Modulation index: μ = (A₁-A₂)/(A₁+A₂)

Frequency Domain:

Shows power distribution across frequencies
Carrier at center frequency fc
Upper sideband at fc+fm
Lower sideband at fc-fm
Bandwidth = 2fm

Diagram:

Time Domain:                         Frequency Domain:
    ^                                    ^
    |                                    |
A₁  |    /\      /\                      |          Carrier
    |   /  \    /  \                     |             |
Ac  |--/----\--/----\--                  |             |
    |  \    /  \    /                    |   LSB       |       USB
A₂  |   \  /    \  /                     |    |        |        |
    |    \/      \/                      |    |        |        |
    +--------------------------->        +----+---------+--------+------>
        t                                   fc-fm      fc      fc+fm

Mnemonic: "TEF" - Time domain shows Envelope, Frequency domain shows spectral components

Question 2(b) OR [4 marks]

Explain pre-emphasis & de-emphasis circuit.

Answer:

Pre-emphasis and De-emphasis Circuits:

Purpose:

Improve SNR for high-frequency components
Compensate for higher noise in high frequencies
Used primarily in FM systems

Pre-emphasis:

Applied at transmitter
Boosts high-frequency components
Typically +6dB/octave above 2.1kHz
Circuit: High-pass RC network (resistor in series, capacitor in parallel)

De-emphasis:

Applied at receiver
Attenuates high-frequency components
Restores original signal balance
Circuit: Low-pass RC network (resistor in parallel, capacitor in series)

Diagrams:

Pre-emphasis:                    De-emphasis:
    R                                C
+---www---+---+                 +---||---+---+
|         |   |                 |        |   |
Vin       C   Vout              Vin      R   Vout
|         |   |                 |        |   |
+----------   |                 +---------   |
             ---                            ---
              -                              -

Frequency response:

    ^
    |        Pre-emphasis
Gain|          /
    |         /
    |        /
0dB |-------/
    |      /       De-emphasis
    |     /          \
    |    /            \
    +-------------------->
       2.1kHz           f

Mnemonic: "HIGH-LOW" - HIGHer frequencies boosted at transmitter, LOWered at receiver

Question 2(c) OR [7 marks]

Compare narrowband FM and wideband FM.

Answer:

Comparison of Narrowband FM and Wideband FM:

Parameter	Narrowband FM	Wideband FM
Modulation Index (β)	β << 1 (typically <0.5)	β >> 1 (typically >5)
Bandwidth	2fm (twice message bandwidth)	2fm(β+1) (Carson's rule)
Significant Sidebands	Only first pair of sidebands	Multiple sidebands
Applications	Mobile communication, two-way radio	FM broadcasting, high-fidelity audio
Signal Quality	Lower fidelity, less noise immunity	Higher fidelity, better noise immunity
Power Efficiency	Higher	Lower
Spectrum Utilization	Efficient	Less efficient
Circuit Complexity	Simpler	More complex

Bandwidth calculation:

Narrowband FM: BW = 2fm
Wideband FM: BW = 2fm(β+1) (Carson's rule)

Spectrum diagram:

Narrowband FM:                    Wideband FM:
    ^                                 ^
    |                                 |
    |            |                    |
    |        |   |   |                |   | | | | | | | | | |
    |    |   |   |   |   |            | | | | | | | | | | | | | |
    +---------------------->          +-------------------------->
       fc-fm  fc  fc+fm                  fc-5fm    fc    fc+5fm

Mnemonic: "BASPCB" - Bandwidth, Applications, Sidebands, Power, Complexity, Beta

Question 3(a) [3 marks]

Define any FOUR characteristics of radio receiver.

Answer:

Characteristics of Radio Receiver:

Sensitivity:
- Ability to amplify weak signals
- Measured in microvolts (μV)
- Typically 1-10μV for good receivers
Selectivity:
- Ability to separate desired signal from adjacent channels
- Determined by bandwidth of IF amplifier
- Measured in dB at specific frequency offsets
Fidelity:
- Accuracy in reproducing original signal
- Depends on bandwidth and distortion
- Measured as frequency response flatness
Image Frequency Rejection:
- Ability to reject signals at image frequency (fi = fs ± 2fIF)
- Measured in dB
- Higher values indicate better performance

Additional characteristics:

Signal-to-noise ratio (SNR)
Automatic gain control (AGC) range
Dynamic range

Mnemonic: "SFID" - Sensitivity, Fidelity, Image rejection, selectivity Determines quality

Question 3(b) [4 marks]

Explain Diode Detector circuit.

Answer:

Diode Detector Circuit:

Purpose:

Extracts original message signal from AM wave
Also called envelope detector

Circuit components:

Diode: Rectifies AM signal
RC network: Filters carrier frequency
R & C values: RC >> 1/fc and RC << 1/fm

Operation:

Diode conducts during positive half-cycles
Capacitor charges to peak value
Capacitor discharges through resistor
RC time constant critical for proper demodulation

Diagram:

          D
        +-----|>|----+
        |            |
Input   |            C    R     Output
AM      |            |    |      
        +------------+----+-----+
                     |          |
                    ---        ---
                     -          -

Waveforms:

Input:                  After Diode:            Output:
    /\      /\              /\      /\              _____
   /  \    /  \            /  \    /  \            /     \
  /    \  /    \    →     /    \  /    \    →     /       \
 /      \/      \        /      \/      \

Limitations:

Distortion for high modulation index
Poor performance at low signal levels

Mnemonic: "DRCO" - Diode Rectifies, Capacitor holds peaks, Output follows envelope

Question 3(c) [7 marks]

Draw and explain block diagram of super heterodyne receiver.

Answer:

Super Heterodyne Receiver:

Block Diagram:

+--------+    +-------+    +------+    +-----+    +--------+    +--------+    +--------+
| Antenna|--->| RF    |--->| Mixer|--->| IF  |--->|Detector|--->| Audio  |--->|Speaker |
|        |    | Amp   |    |      |    | Amp |    |        |    | Amp    |    |        |
+--------+    +-------+    +------+    +-----+    +--------+    +--------+    +--------+
                              ^
                              |
                        +------------+
                        | Local      |
                        | Oscillator |
                        +------------+

Function of each block:

RF Amplifier:
- Amplifies weak RF signals
- Provides selectivity
- Improves signal-to-noise ratio
Local Oscillator:
- Generates stable frequency fLO
- fLO = fRF + fIF (for high-side injection)
- Tuned with RF amplifier
Mixer:
- Combines RF signal with local oscillator
- Produces sum and difference frequencies
- Difference frequency = IF (intermediate frequency)
IF Amplifier:
- Fixed frequency amplification (typically 455kHz for AM)
- Provides most of receiver gain and selectivity
- Multiple stages for better performance
Detector:
- Demodulates IF signal
- Extracts original message signal
- Diode detector for AM, discriminator for FM
Audio Amplifier:
- Amplifies demodulated signal
- Drives speaker or headphones

Working principle:

Converts any RF frequency to fixed IF for efficient amplification
IF frequency = |fRF - fLO|

Advantages:

Better selectivity and sensitivity
Stable gain at all frequencies
Reduced tracking problems

Mnemonic: "RLMIDS" - RF amp, Local oscillator, Mixer, IF amp, Detector, Speaker

Question 3(a) OR [3 marks]

Describe AGC principle and its application in Radio receiver.

Answer:

AGC (Automatic Gain Control) Principle:

Definition:

Circuit that automatically adjusts receiver gain based on signal strength
Maintains constant output level despite varying input signals

Working principle:

Detects received signal strength
Generates control voltage proportional to signal
Applies negative feedback to reduce gain for strong signals
Increases gain for weak signals

Application in Radio Receiver:

Prevents overloading: Protects against strong signal distortion
Compensates fading: Maintains constant volume during signal fading
Controls IF amplifier: Primarily applied to IF stages
Improves dynamic range: Handles wide range of signal strengths

Types:

Simple AGC: Direct feedback from detector
Delayed AGC: Only activates above threshold level
Amplified AGC: Uses additional amplifier for better control

Diagram:

     +-------+    +------+    +-----+    +--------+
     | RF    |--->| Mixer|--->| IF  |--->|Detector|---> Audio
     | Amp   |    |      |    | Amp |    |        |
     +---|---+    +------+    +-|---+    +----|---+
         |                      |             |
         |                      |         +--------+
         |                      |---------|  AGC   |
         |                                | Circuit|
         +--------------------------------|        |
                                          +--------+

Mnemonic: "FADS" - Fading compensation, Automatic adjustment, Dynamic range, Signal consistency

Question 3(b) OR [4 marks]

Write short-note on intermediate frequency

Answer:

Intermediate Frequency (IF):

Definition:

Fixed frequency to which incoming RF signal is converted in superheterodyne receivers
Result of mixing (heterodyning) RF signal with local oscillator

Standard IF values:

AM radio: 455 kHz
FM radio: 10.7 MHz
TV receivers: 38-41 MHz

Importance:

Consistent gain: Amplifiers operate at fixed frequency
Better selectivity: Narrowband filters at fixed frequency
Simplified design: Easier to design efficient fixed-frequency stages

Selection criteria:

High enough to provide good image rejection
Low enough for practical filter Q and gain
Should avoid harmonics of common signals

Image frequency calculation:

High-side injection: fimage = fRF + 2fIF
Low-side injection: fimage = fRF - 2fIF

Diagram:

   Original      IF Stage      Audio
   Spectrum        Fixed       Output
     |  |           |  |          |
     V  V           V  V          V
+----------+    +----------+    +-----+
|  Mixer   |--->|    IF    |--->| Det |
+----------+    +----------+    +-----+
      ^
      |
+------------+
|  Local     |
| Oscillator |
+------------+

Mnemonic: "CIGS" - Conversion, Improved selectivity, Gain stability, Simplified design

Question 3(c) OR [7 marks]

Explain phase discriminator circuit for FM detection.

Answer:

Phase Discriminator for FM Detection:

Purpose:

Converts frequency variations in FM signal to amplitude variations
Demodulates FM signal to recover original message

Circuit components:

Center-tapped transformer
Two diodes (D1 and D2)
RC filter network
Phase-shifting network (L-C circuit)

Working principle:

Input FM signal splits into two paths
Reference path goes directly to center tap
Phase-shifted path passes through LC network
Phase shift varies with frequency deviation
Two diodes produce voltages proportional to phase difference
Output voltage varies with input frequency

Circuit diagram:

                      D1
              +------|>|------+
              |               |
              |               R1
              |               |
              |               |
 FM Input     |               +---+
 +------+     |                   |
 |      |-----+                   +--- Output
 +------+     |                   |
              |               +---+
              |               |
              |               R2
              |               |
              +------|<|------+
                      D2

Characteristics:

Linear response over moderate frequency range
Balanced design reduces amplitude variations
High sensitivity to frequency changes
Limitations at extreme frequency deviations

S-curve response:

    ^
    |              /
    |             /
    |            /
 0V +------------
    |          /
    |         /
    |        /
    +----------------->
       fc-Δf  fc  fc+Δf

Mnemonic: "PSDO" - Phase shift Demodulates, Signal frequency determines Output

Question 4(a) [3 marks]

Compare analog and digital communication techniques

Answer:

Comparison of Analog vs. Digital Communication:

Parameter	Analog Communication	Digital Communication
Signal	Continuous waveform	Discrete binary values
Bandwidth	Less bandwidth required	More bandwidth required
Noise Immunity	Poor, noise accumulates	Excellent, error correction possible
Power Efficiency	Less efficient	More efficient
Quality	Degrades with distance	Maintains quality until SNR threshold
Multiplexing	FDM primarily used	TDM primarily used
System Complexity	Simpler	More complex
Cost	Lower	Higher but decreasing
Examples	AM/FM radio, analog TV	Mobile networks, digital TV, internet

Mnemonic: "BNPQ MCE" - Bandwidth, Noise immunity, Power, Quality, Multiplexing, Complexity, Efficiency

Question 4(b) [4 marks]

Explain Adaptive delta modulation with its application.

Answer:

Adaptive Delta Modulation (ADM):

Definition:

Improved version of Delta Modulation (DM)
Uses variable step size adjusted to signal slope

Working principle:

Compares input signal with predicted value
Outputs binary 1 or 0 based on comparison
Adjusts step size based on consecutive bits
Increases step size for rapid changes
Decreases step size for slow changes

Advantages over Delta Modulation:

Reduces slope overload distortion
Minimizes granular noise
Better dynamic range
Lower bit rate for same quality

Diagram:

                        +-------+
                        |       |
                        | Step  |<--+
Input                   | Size  |   |
  +---+    +---+        | Logic |   |
  |   |--->|+/-|------->|       |   |
  +---+    +---+        +-------+   |
    ^        |              |       |
    |        |              V       |
    |      +---+         +---+      |
    +------|   |<--------| Δ |------+
           +---+         +---+
           Integrator

Applications:

Speech transmission: Voice over digital networks
Audio compression: Music storage and transmission
Telemetry systems: Remote data collection
Military communications: Secure transmission

Mnemonic: "VSOG" - Variable Step size Overcomes Granular noise & slope overload

Question 4(c) [7 marks]

Draw & explain block diagram of PCM system.

Answer:

Pulse Code Modulation (PCM) System:

Block Diagram:

                  +-------+    +----------+    +---------+    +--------+
                  |       |    |          |    |         |    |        |
Input signal ---->|Sample |--->|Quantizer |--->|Encoder  |--->|Channel |
                  |& Hold |    |          |    |         |    |        |
                  +-------+    +----------+    +---------+    +--------+
                                                                  |
                                                                  V
                  +--------+    +---------+    +---------+    +--------+
                  |        |    |         |    |         |    |        |
Output signal <---|Low Pass|<---| DAC     |<---|Decoder  |<---| Buffer |
                  |Filter  |    |         |    |         |    |        |
                  +--------+    +---------+    +---------+    +--------+

Transmitter components:

Sample & Hold:
- Samples analog signal at regular intervals
- Nyquist rate (fs ≥ 2fmax)
- Holds value until next sample
Quantizer:
- Divides amplitude range into discrete levels
- Maps each sample to nearest level
- Introduces quantization error
Encoder:
- Converts quantized levels to binary code
- n-bit encoder gives 2^n quantization levels
- Common formats: 8-bit, 16-bit

Receiver components:

Decoder:
- Converts binary to quantized levels
- Reverses encoder operation
Digital-to-Analog Converter (DAC):
- Converts discrete levels to analog values
- Produces staircase approximation of signal
Low-Pass Filter:
- Smooths staircase output
- Removes high-frequency components
- Reconstructs original waveform

Key characteristics:

Sampling rate: Typically 8 kHz (voice), 44.1 kHz (CD audio)
Resolution: 8-bit (256 levels) to 24-bit (16.8M levels)
Bit rate = Sampling rate × bits per sample

Mnemonic: "SQEC-DFL" - Sample, Quantize, Encode, Channel - Decode, Filter, Listen

Question 4(a) OR [3 marks]

Explain quantization process and its necessity.

Answer:

Quantization Process and its Necessity:

Definition:

Process of mapping continuous amplitude values to discrete levels
Second step in analog-to-digital conversion after sampling

Process:

Divide amplitude range into finite number of levels
Assign each sample to nearest quantization level
Represent each level with binary code
Quantization levels = 2^n (n = number of bits)

Types:

Uniform quantization: Equal step size throughout range
Non-uniform quantization: Variable step size (smaller for lower amplitudes)
Mid-tread quantization: Zero is a valid level
Mid-rise quantization: Zero falls between levels

Necessity:

Digital representation: Enables conversion to binary format
Storage efficiency: Allows finite storage of analog signals
Processing capability: Enables digital signal processing
Transmission benefits: Facilitates error correction and encryption

Quantization error:

Difference between actual and quantized value
Maximum error = ±Q/2 (where Q = step size)
Signal-to-quantization-noise ratio: SQNR = 6.02n + 1.76 dB

Diagram:

  ^
  |                    Quantized
  |   Original         Output
  |    Signal          /|
  |      /\           / |
  |     /  \         /  |
  |    /    \       /   |
  |   /      \     /    |
  |  /        \   /     |
  | /          \ /      |
  +--------------------------->
                Time

Mnemonic: "DEBS" - Digitization Enables Binary Storage

Question 4(b) OR [4 marks]

Explain PCM receiver.

Answer:

PCM Receiver:

Block Diagram:

                  +--------+    +---------+    +---------+    +--------+
                  |        |    |         |    |         |    |        |
Digital PCM  ---->| Buffer |--->| Decoder |--->|   DAC   |--->|Low Pass|---> Output Signal
  Input           |        |    |         |    |         |    | Filter |
                  +--------+    +---------+    +---------+    +--------+

Components and their functions:

Buffer:
- Temporarily stores received PCM data
- Compensates for timing variations
- Provides protection against jitter
Decoder:
- Converts binary code to quantized amplitude levels
- Detects and corrects transmission errors (if error coding used)
- Outputs discrete amplitude values
Digital-to-Analog Converter (DAC):
- Converts digital values to analog voltage levels
- Creates staircase approximation of original signal
- Resolution determined by bit depth (2^n levels)
Low-Pass Filter:
- Smooths the staircase waveform
- Removes high-frequency components
- Reconstructs continuous analog signal

Waveforms in PCM Receiver:

Digital Input      Decoded Values       DAC Output          Final Output
 1001              ----                  _                    /\
 0110              -  -                _| |_                 /  \
 1010       →      -- -        →      _|   |_       →       /    \
 0101              - - -             _|     |_             /      \

Performance factors:

SNR: Determined by quantization bits (6.02n + 1.76 dB)
Bandwidth: Depends on sampling rate and filter characteristics
Distortion: Related to quantization error

Mnemonic: "BDFL" - Buffer stores, Decoder converts, Filter smooths, Listen to output

Question 4(c) OR [7 marks]

What is sampling? Explain types of sampling in brief.

Answer:

Sampling:

Definition: Sampling is the process of converting a continuous-time signal into a discrete-time signal by taking measurements (samples) at regular time intervals.

Mathematical expression: x[n] = x(nTs), where n = 0, 1, 2...

x[n] is discrete-time sample
x(t) is continuous-time signal
Ts is sampling period (1/fs)

Nyquist Theorem:

Sampling frequency (fs) must be at least twice the highest frequency component (fmax) in the signal
fs ≥ 2fmax
Prevents aliasing (distortion due to overlap of spectrum)

Types of Sampling:

Type	Description	Characteristics
Ideal Sampling	Instantaneous samples at regular intervals	- Theoretical concept - Represented by impulse train - Infinite bandwidth required
Natural Sampling	Signal multiplied by pulse train with finite width	- Samples have same shape as signal - Width determined by sampling pulse - Used in analog systems
Flat-Top Sampling	Sample-and-hold technique	- Holds sampled value until next sample - Creates staircase approximation - Common in practical systems

Sampling Rates:

Under-sampling: fs < 2fmax (causes aliasing)
Critical sampling: fs = 2fmax (minimum required rate)
Over-sampling: fs > 2fmax (improves reconstruction quality)

Diagram:

Original Signal:     /\/\/\/\/\/\/\/\

Ideal Sampling:      |  |  |  |  |  |

Natural Sampling:    ▓  ▓  ▓  ▓  ▓  ▓

Flat-top Sampling:   ▔▔  ▔▔  ▔▔  ▔▔  ▔▔

Mnemonic: "INF" - Ideal (impulses), Natural (pulse-shaped), Flat-top (staircase)

Question 5(a) [3 marks]

List the need of Multiplexing.

Answer:

Need for Multiplexing:

Need	Description
Bandwidth Utilization	Efficiently uses available transmission bandwidth
Cost Reduction	Shares expensive transmission medium among multiple users
Infrastructure Optimization	Reduces physical connections and hardware requirements
Spectrum Efficiency	Maximizes use of limited frequency spectrum
Network Capacity	Increases number of channels/users on single medium
Flexibility	Allows dynamic allocation of resources based on demand

Mnemonic: "BCSINF" - Bandwidth, Cost, Spectrum, Infrastructure, Network capacity, Flexibility

Question 5(b) [4 marks]

Explain working of DPCM.

Answer:

Differential Pulse Code Modulation (DPCM):

Definition:

Enhanced version of PCM that encodes difference between current and predicted sample
Exploits correlation between adjacent samples to reduce bit rate

Block Diagram:

                  +------+     +----------+    +---------+
                  |      |     |          |    |         |
Input signal ---->| ADC  |--+->|Quantizer |--->|Encoder  |---> DPCM Output
                  |      |  |  |          |    |         |
                  +------+  |  +----------+    +---------+
                            |        ^
                            |        |
                            v        |
                        +---------+  |
                        |Predictor|--+
                        +---------+

Working principle:

Current sample is predicted based on previous sample(s)
Only the difference (error) between actual and predicted value is encoded
Smaller difference requires fewer bits than full amplitude
Predictor uses previous reconstructed values for prediction

Advantages:

Reduced bit rate: Typically 25-50% lower than PCM
Better SNR: For same bit rate as PCM
Correlation utilization: Exploits signal redundancy

Limitations:

Error propagation: Errors affect subsequent samples
Complexity: More complex than simple PCM
Signal dependency: Performance varies with signal characteristics

Mnemonic: "PDQE" - Predict sample, Difference calculated, Quantize error, Encode result

Question 5(c) [7 marks]

The binary data 1011001 is to be transmitted using following line coding techniques: (i) Unipolar RZ and NRZ (ii) Polar RZ and NRZ (iii) AMI (iv) Manchester. Draw all the waveforms.

Answer:

Line Coding of Binary Data: 1011001

Waveforms:

Binary Data:   1   0   1   1   0   0   1
              _   _   _   _   _   _   _

1. Unipolar NRZ:
              ▔▔▔   ▔▔▔▔▔▔   ▔▔▔
              ___ ▔▔▔ _______ ▔▔▔

2. Unipolar RZ:
              ▔ _ ▔ _ ▔ ▔ _ _ _ ▔ _
              _ _ _ _ _ _ _ _ _ _ _ _

3. Polar NRZ:
              ▔▔▔   ▔▔▔▔▔▔   ▔▔▔
              ___ ▔▔▔ _______ ▔▔▔

4. Polar RZ:
              ▔ _ _ _ ▔ ▔ _ _ _ ▔ _
              _ ▔ _ _ _ _ ▔ ▔ _ _ _

5. AMI:
              ▔ _   ▔ _ _ _ ▔ _
              _ _ ▔ _ _ _ _ _ _ _ _

6. Manchester:
              ▔▁ ▁▔ ▔▁ ▔▁ ▁▔ ▁▔ ▔▁
              _ _ _ _ _ _ _ _ _ _ _ _

Characteristics of Each Coding:

Coding Technique	Description	Advantages	Disadvantages
Unipolar NRZ	1 = high voltage 0 = zero voltage No return to zero	Simple implementation	DC component, no clock recovery
Unipolar RZ	1 = high for half bit 0 = zero voltage Returns to zero	Self-clocking	Requires more bandwidth
Polar NRZ	1 = positive voltage 0 = negative voltage No return to zero	No DC component	Poor clock recovery
Polar RZ	1 = positive for half bit 0 = negative for half bit Returns to zero	Self-clocking, no DC component	Requires more bandwidth
AMI	1 = alternating +/- voltage 0 = zero voltage	No DC component, error detection	Long strings of zeros problematic
Manchester	1 = transition low to high 0 = transition high to low	Self-clocking, no DC component	Requires double bandwidth

Mnemonic: "UPRMA" - Unipolar, Polar, Return-to-zero, Manchester, AMI line coding techniques

Question 5(a) OR [3 marks]

Explain polar RZ and NRZ format

Answer:

Polar RZ and NRZ Line Coding:

Polar NRZ (Non-Return to Zero):

Binary 1: Positive voltage (+V) for entire bit duration
Binary 0: Negative voltage (-V) for entire bit duration
Signal remains at level during entire bit period
No transition to zero between consecutive similar bits

Characteristics of Polar NRZ:

Bandwidth efficiency: Requires minimum bandwidth
DC component: Zero average for equal 1s and 0s
Clock recovery: Poor for long sequences of same bit
Error detection: No inherent capability

Polar RZ (Return to Zero):

Binary 1: Positive voltage (+V) for half bit, zero for remainder
Binary 0: Negative voltage (-V) for half bit, zero for remainder
Signal returns to zero during each bit period

Characteristics of Polar RZ:

Bandwidth: Requires twice the bandwidth of NRZ
Self-clocking: Better clock recovery
Power requirement: Higher than NRZ
Error detection: No inherent capability

Waveform Comparison:

Binary Data:   1   0   1   1   0   0   1
              _   _   _   _   _   _   _

Polar NRZ:    ▔▔▔   ▔▔▔▔▔▔   ▔▔▔
              ___ ▔▔▔ _______ ▔▔▔

Polar RZ:     ▔ _ _ _ ▔ ▔ _ _ _ ▔ _
              _ ▔ _ _ _ _ ▔ ▔ _ _ _

Mnemonic: "HZRT" - Half bit active + Zero Return in RZ, full Time in NRZ

Question 5(b) OR [4 marks]

Explain delta modulation in brief.

Answer:

Delta Modulation (DM):

Definition:

Simplest form of differential encoding
Encodes only the sign of difference between current and previous sample
Single bit per sample for transmission (1 or 0)

Block Diagram:

                    +-----+       Encoded
 Input      +---+   |     |      Bitstream
 Signal --->|+/-|--->  C  |---------->
            +---+   |     |
              ^     +-----+
              |        |
              |        v
            +---+    +---+
            |   |<---|+/-|
            +---+    +---+
          Integrator   Step Size

Working principle:

Compare input signal with predicted value (from integrator)
If input > predicted: Output = 1, increase predicted value
If input < predicted: Output = 0, decrease predicted value
Step size determines how much predicted value changes

Advantages:

Simple implementation: Minimal hardware
Low bit rate: 1 bit per sample
Robust: Relatively immune to channel noise

Limitations:

Slope overload: Cannot track rapid signal changes
Granular noise: Oscillations around steady signals
Limited resolution: Quality depends on step size and sampling rate

Waveforms:

Original:      /\/\/\/\

Reconstructed: /\/\/\/\
               (Staircase approximation)

Binary output: 1101001011

Mnemonic: "1BSG" - 1 Bit per Sample, Slope overload and Granular noise limitations

Question 5(c) OR [7 marks]

Explain PCM-TDM system.

Answer:

PCM-TDM System:

Definition:

Combined system using Pulse Code Modulation (PCM) with Time Division Multiplexing (TDM)
Multiple analog channels converted to digital PCM, then multiplexed in time

Block Diagram:

                 +-------+     +--------+     +---------+
 Channel 1 ----->| PCM 1 |---->|        |     |         |
                 +-------+     |        |     |         |
                 +-------+     |        |     |         |     Multiplexed
 Channel 2 ----->| PCM 2 |---->|  Time  |---->|  Frame  |---> PCM-TDM
                 +-------+     |        |     | Format  |     Output
                               |  MUX   |     |         |
                 +-------+     |        |     |         |
 Channel N ----->| PCM N |---->|        |     |         |
                 +-------+     +--------+     +---------+

PCM Process for Each Channel:

Sampling: Each channel sampled at fs ≥ 2fmax
Quantization: Samples assigned to discrete levels
Encoding: Quantized values converted to binary code

TDM Frame Structure:

Frame consists of one sample from each channel
Frame includes synchronization bits/word
Frame rate equals sampling rate (fs)
Bit rate = fs × N × n (N = channels, n = bits/sample)

Typical Parameters:

Voice channels: 8 kHz sampling, 8 bits/sample
T1 system: 24 channels, 1.544 Mbps
E1 system: 30 channels, 2.048 Mbps

Advantages:

Efficient transmission: Single high-speed link
Digital benefits: Noise immunity, regeneration
Flexibility: Easy to add/drop channels

Applications:

Telephone networks: Digital transmission systems
Digital audio: Broadcasting and recording
Satellite communications: Multiple channel transmission

Diagram of TDM Frame:

   |<-------------- One TDM Frame -------------->|
   +-----+-----+-----+-----+-----+       +-----+
   | Sync| Ch1 | Ch2 | Ch3 | Ch4 | ..... | ChN |
   +-----+-----+-----+-----+-----+       +-----+

Mnemonic: "MSQT" - Multiplex, Sample, Quantize, Transmit